1. Field of the Invention
The present invention relates to a brake disc having a corrugated outer periphery suitable as a brake disc for a two-wheeled motor vehicle.
2. Description of the Background Art
As a brake disc for a two-wheeled motor vehicle, a floating brake disc widely becomes commercially practical. The floating brake disc includes a hub disc fixed to a wheel, a braking disc disposed on an outer side of the hub disc coaxially, and a connection pin that connects the hub disc and the braking disc in a floating manner. As such a floating brake disc, there are a brake disc having a circular outer periphery, and a brake disc having a corrugated outer periphery in which a plurality of recesses are formed in the outer periphery at a distances from one another in the circumferential direction. The brake disc having the corrugated outer periphery is light in weight and has excellent mud-drainage and excellent design and thus, it is on board of not only an off-road motorcycle but also an on-road motorcycle (e.g., see patent document 1).
In the meantime, as vibration at the time of braking operation of a disc brake device, brake judder in a low frequency region is known other than brake squeal in a high frequency region. The brake judder is abnormal vibration at the time of the braking operation, and a driver feels the abnormal vibration of a handle bar or a brake lever as the brake judder. If a poor driver encounters the brake judder for the first time, there is an adverse possibility that the driver erroneously operates the handle bar or the brake and thus, it is desired to reliably prevent the brake judder to enhance the safety of the vehicle.
It is known that the brake judder phenomenon in the brake disc having the circular outer periphery is caused by fluctuation in the brake torque and the brake liquid pressure due to the manufacturing precision of the brake disc such as the thickness precision of the brake disc and the flatness of the brake disc. Therefore, the brake disc having the corrugated outer periphery is also designed such that the manufacturing precision of the brake disc is enhanced to prevent the brake judder.    [Patent document 1] International Patent Publication No. 2004-042247
In the brake disc having the corrugated outer periphery, however, even if the manufacturing precision of the brake disc is enhanced in the same level as the brake disc having the circular outer periphery, the brake judder is generated in some cases. In such a case, it is a situation in which the design is changed by trial and error, e.g., sizes of various portions of the brake disc are changes and a brake disc in which a brake judder is not generated is produced. It is required to find a cause of the brake judder and to produce a brake disc that can reliably prevent the brake judder. As a result of the design made by trial and error, a floating brake disc in which the number of recesses formed in the outer periphery is 24, the number of connection pins is 7 and the number of mounting holes is 5, and these numbers are relative primes has become commercially practical.